Topic unpopulated once client reconnects

[jkominiakgps]

I have a client, using version 4.2.5, with a subscription and once the client reconnects, the topic property is no longer populated when a message is received. Inspecting the packet of the message event shows the topic as empty as well. To recreate the issue, the client just needs to be kicked from the broker it is connected to by either kicking out that specific client or by restarting the broker. Both scenarios result in the topic not populating.

Code:

client.on("connect", () => {
    console.log("Connection Success");
    client.subscribe("tmp/#", { qos: 1 }, (err) => {
      console.log(err || "Subscribe Success");
    });
  });

  client.on("message", (topic, message, packet) => {
    console.log("Received from", topic);
    console.log("Received packet", packet);
  });

  client.on("reconnect", () => {
    console.log("Reconnecting");
  });

  client.on("error", (err) => {
    console.log("Error:", err);
  });

Log:

Connection Success
Subscribe Success
Received from tmp/hello
Received packet Packet {
  cmd: 'publish',
  retain: false,
  qos: 0,
  dup: false,
  length: 33,
  topic: 'tmp/hello',
  payload: <Buffer 7b 0d 0a 20 20 22 68 65 6c 6c 6f 20 77 6f 72 6c 64 22 0d 0a 7d>
}
Received from tmp/hello
Received packet Packet {
  cmd: 'publish',
  retain: false,
  qos: 0,
  dup: false,
  length: 33,
  topic: 'tmp/hello',
  payload: <Buffer 7b 0d 0a 20 20 22 68 65 6c 6c 6f 20 77 6f 72 6c 64 22 0d 0a 7d>
}
Reconnecting
Connection Success
Subscribe Success
Received from tmp/hello
Received packet Packet {
  cmd: 'publish',
  retain: false,
  qos: 0,
  dup: false,
  length: 36,
  topic: 'tmp/hello',
  payload: <Buffer 7b 0d 0a 20 20 22 68 65 6c 6c 6f 20 77 6f 72 6c 64 22 0d 0a 7d>,
  properties: { topicAlias: 1 }
}
Received from 
Received packet Packet {
  cmd: 'publish',
  retain: false,
  qos: 0,
  dup: false,
  length: 27,
  topic: '',
  payload: <Buffer 7b 0d 0a 20 20 22 68 65 6c 6c 6f 20 77 6f 72 6c 64 22 0d 0a 7d>,
  properties: { topicAlias: 1 }
}

AB#8825351

[akiroz]

I'm experiencing the same issue with async-mqtt: 2.6.1 which depends on mqtt: 4.1.0.

I'm guessing this has something to do with the MQTTv5 implementation?

===============================================

EDIT: I've done some further digging and it seems to be a broker issue. After some packet capturing via Wireshark, it seems the topic is missing from the incoming MQTT packet. I'm using the EMQ X broker (4.2.10).

[akiroz]

Okay I've finally got to the bottom of this, it seems to be a MQTT.js bug in the reconnect logic.

Some context: MQTTv5 supports a feature called Topic Aliasing where topics can be replaced by a short numeric ID after being sent once to reduce packet size.

During a fresh connect from MQTT.js, the CONNECT packet's Topic Alias Maximum property is set to 0 by default which indicates that aliasing is not supported:

However, when MQTT.js reconnects, the CONNECT packet's Topic Alias Maximum property is set to 0xFFFF causing the broker to start using topic aliases for subsequent messages:

[akiroz]

So it seems like the topicAliasMaximum property is actually set when handling the first CONNACK packet provided by the server, is this expected behavior when users are expected to handle topic de-aliasing manually?

https://github.com/mqttjs/MQTT.js/blob/37b12cb94737acddfe0fa016e013516ba3b9a031/lib/client.js#L1174

I can work around this issue by deleting properties after connect:

client.on("connect", () => {
    client.options.properties = {};
});

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[jkominiakgps]

Appears to be resolved with version 4.3.7